Computer devices are becoming ever smaller and full computing functionality can be found on phones and smart phones and other personal digital assistants (PDAs). As the computer devices become smaller so the various features of the computer devices also become smaller. This includes a condition for smaller input systems for the user to enter inputs into the device. One such input system is an optical navigation device. Many computer devices, large and small, are equipped with optical navigation devices, such as a mouse. However, with the smaller computer devices, reducing the size of the optical navigation device can often be restrictive and problematic.
One problem which remains unresolved is the lighting levels of optical navigation devices when operating in high ambient light conditions. In a PC a mouse is usually operated pointing towards a surface (e.g. a desk, mouse mat, etc.) as a result little stray or ambient light reaches the sensor. However, in a fingermouse on a mobile phone the mouse surface usually points upwards and as a result can receive a large amount of ambient stray light or sunlight.
Levels of sunlight can vary depending on weather, the location (latitude) of the user, seasons, etc. It is not uncommon for the levels of sunlight to exceed the dynamic range of existing mice sensors resulting in a saturated image on the sensor. If all the pixels are saturated or “clipped” it is not possible to recognize any movement or finger ridges on the mousing surface and as such the optical navigation function does not function. This is a severe problem for a mobile or smart phone as the user is not able to access any navigational aids, such as icons, lists etc. in high ambient light levels.
A number of approaches to the problem of high ambient light levels and the saturation of the sensor have been proposed. These include optical filtering; electronic exposure control; offset compensation; pixel skimming; and digital pixel offset cancelation techniques.
Optical filtering includes adding a filter to the fingermouse module which will stop or attenuate visible light but pass infrared light. Due to the nature of sunlight certain energy bands pass through the filter and are detected by the sensor, leading again to saturation of the pixels and prevention of operation of the fingermouse. This can be improved by carefully controlling and selecting the cut-off wavelength of the filter. However, this does not completely address the problem.
Electronic exposure control is a common technique used in CMOS sensors where the pixel is controlled with respect to how much time the pixel is sensitive to light. This is done by controlling the time the pixel is in the reset state. In high light levels the pixel is kept in “reset” for a longer duration resulting in a short integration period and under low light levels the pixel is kept in “reset” for a shorter duration resulting in a long integration time. While this is generally acceptable for image sensors, the technique causes problems for a fingermouse. This is due to the fact that the mouse sensor has a “global shutter”, where all pixels are exposed and read-out simultaneously to avoid distortion of the image due to movement on the sensor. Further, a low-voltage operation of some mice sensors causes problems with offsetting. Both of these aspects may hinder standard operation of electronic exposure control in a fingermouse.
CMOS sensors commonly adopt techniques to compensate for offset. Typically, these operate by taking one “dark” measurement when the pixel is in a reset mode and another with the image exposed and comparing the measurements. Systematic offsets are common to both measurements, so by subtracting the two measurements, the offsets are removed. This technique is generally referred to as “Double Sampling”. There are two variants of double sampling, namely correlated double sampling (CDS) and double sampling (DS). CDS takes the first measurement just after the pixel has come out of reset (i.e. at the start of the integration cycle) and the second measurement at the end of the integration cycle. The disadvantage of this scheme is that it requires the storage of the “dark” measurement for a long period such as one whole frame. This may require the need for a frame store or other similar storage devices which adds to the cost of implementation and is undesirable.
A pixel in a mouse is typically of the order of 30 μm×30 μm and as such is relatively large for a pixel. This is necessary as a mouse needs to operate at high frame rate and therefore needs to collect more light to obtain a reasonable signal. As a result, fully depleted 4T photodiodes are not practical for optical mouse sensors. Even without reset noise from the photodiode, 4T pixels still suffer from offsets and reset noise from other capacitors in the readout chain (notably the sense node capacitance) and employ DS/CDS techniques to overcome these. This would not be satisfactory in a fingermouse for the reasons discussed above.
Pixel skimming is a technique to increase the intra-scene (single image) Dynamic Noise Reduction (DNR) of a pixel and only works with pixels having a transfer gate (i.e. “4T” architecture) as it relies on pulsing the gate to a predetermined value to partially reset the signal on the photodiode. As described earlier, this technique is not available to larger pixels, such as those found on a fingermouse. Variants of this technique apply multiple skimming pulses per image although only some pixels (those with high light levels) are reset.
A currently used technique known as digital pixel offset cancellation is disclosed in U.S. Pat. No. 7,502,061 and is described with reference to FIG. 1. In a PC mouse, the LED on time is changed to control the exposure. On a fingermouse, this technique works very well with low or normal lighting conditions. However, under high levels of ambient light, most of the light on the sensor is from the sun and thus reducing the LED on time to zero does not prevent the pixel from saturating.
Ideally, the integration period ((A) in FIG. 1) should be reduced, however it is not possible to reduce the integration time (A) below the time it takes for the “blackcal” data to be read out (B). This is due to the fact that the (reset) data stored in the pixel analog to digital converter (ADC) would otherwise be over-written by data from the analog-digital conversion of the integrate phase (“Convert” in FIG. 1). To avoid this, it would be necessary to add an extra storage capability to the array, this would require a significant amount of space and as a result would increase the cost, which would be undesirable.